Gamecube is backwards compatible but you can also get component cables for it, not cheap but worth it
hate using gamecube games on wii dont feel proper ^^.
snes can use RGB/Scart as well as n64, and nes.
you can use a scart switch box for the oldies, and a component switch box if you have more than 1 component source.
conneted to my TV at the moment is wii, xbox 360, ps3, xbox, ps2, gamecube, atari jaguar, megadrive, snes, dreamcast and pc
Component on xbox gamecube ps2 wii
HDMI on 360 ps3 PC
RGB on snes megadrive jaguar
VGA for Dreamcast